1. Field of the Invention
This invention relates to methods for compression and expansion of display data, which allow irreversible high efficiency compression to be performed on display data realizing pictures naturally displayed in the RGB form, and which allows reversible compression to be performed on display data of the index form.
This application claims priority on Japanese Patent Application No. 2003-431199, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, the following methods for compression of color image data are known.
(1) Cell Reference Method, and Line Reference Method
These methods effectively work on solid pictures in which the same color codes consecutively emerge before and after display data; however, they cannot raise compression ratios so much with respect to pictures in which same color codes do not consecutively emerge before and after display data. In addition, these methods are incapable of performing irreversible high efficiency compression on image data in the RGB form.
(2) Dictionary Method
In order to increase a rendering factor, this method requires numerous dictionary data. Similar to the cell reference method and line reference method, this method is designed to realize reversible compression; in other words, it is impossible to perform irreversible high efficiency compression on image data in the RGB form.
(3) DCT and Two-dimensional Huffman Coding Based on JPEG (i.e., Joint Photographic Image Coding Experts Group)
This method performs DCT (i.e., discrete cosine transform) on blocks each having 8×8 size, so that distortion may apparently emerge in units of blocks when the compression ratio is increased. This method is designed to basically perform sine approximation; hence, it is incapable of approximating edge-enhanced portions of animated images and the like. In addition, two-dimensional Huffman coding requires complicated Huffman tables, wherein table reference processes are difficult to be realized in the hardware. Furthermore, this method is incapable of performing reversible compression on image data having the index form realized by color palette conversion.
(4) Wavelet Transform and Arithmetic Coding (Based on JPEG2000)
In order to realize image compression and two-dimensional wavelet transform, this method requires buffers whose storage sizes match pixel sizes; hence, it is difficult to be realized in the hardware. In addition, this method requires a relatively long time to perform transform. Furthermore, arithmetic coding requires complicated calculations. In particular, when arithmetic coding is performed in units of bit planes in JPEG2000, it requires a relatively long time in decoding. Similar to DCT, the wavelet transform is realized using the correlation between images; hence, it does not suit reversible compression of image data.
In order to cope with the drawbacks of the conventionally known compression methods, we, the inventors, have filed Japanese Patent Application No. 2002-234984, which is now published as Japanese Patent Application Publication No. 2004-80154, which teaches irreversible high efficiency compression of image data in the RGB form and reversible compression of image data in the index form.